The invention relates to a molding tool for producing hot-formed components.
In present-day automotive engineering, vehicle passenger comfort is increasingly improved by employing optional equipment. The latter includes many electromechanical components such as sensors, motors, actuators, and serves to facilitate the driving task for the driver. However, as comfort increases, so does the vehicle weight. In order to counteract the latter, attempts at designing the structural components of the bodywork in a weight-reduced manner are made in the prior art.
The structural components of the bodywork are not only relevant to the stability of the vehicle, but also play a decisive role in terms of crash safety. In order for this conflict of aims between reducing the component weight of structural components and at the same time maintaining or implementing, respectively, high mechanical characteristics to be resolved, it has proven successful in the past for structural parts to be produced by hot-forming. Hot-forming processes are also referred to in the literature as form-hardening or press-hardening.
Two fundamentally dissimilar methods are known for producing form-hardened components, in particular for producing bodywork components. In the case of the direct hot-forming method, a blank is initially heated in a furnace to a temperature above the austenitizing temperature of the steel, and is subsequently simultaneously formed and cooled, that is to say form-hardened, in a tool. In the indirect hot-forming method, a fully formed and trimmed component from steel is first generated from a blank by cold-forming. The component is then heated in a heating plant to a temperature above the austenitizing temperature of the steel, and is subsequently form-hardened by rapid cooling in a tool. In both hot-forming methods, the blank or an already fully formed and trimmed component from steel, subsequent to heating to the austenitizing temperature, is thermomechanically formed in the tool, wherein thermomechanical forming is performed at a temperature above the Ac3 austenitizing temperature (approx. 830° C.), preferably between 900 and 1100° C. Cooling of the formed workpieces is performed by means of a cooling unit which is located in a closed tool body. For this reason, components having particularly advanced mechanical properties, in particular having high strengths, may be generated.
Patent document DE 19723655 B4 describes a method for producing sheet-steel panel products by heating a cut-to-measure steel-sheet panel, hot-forming the steel-sheet panel in a pair of tools, hardening the product formed by rapid cooling from an austenitizing temperature while the panel continues to be held in the pair of tools, and subsequent processing of the product.
Proceeding from this prior art, it is an object of the present invention to provide a molding tool for producing components, in particular vehicle components made of sheet metal, in which particularly good mechanical characteristics are accomplished and the component weight is reduced at the same time.
This and other objects are achieved in accordance with embodiments of the invention.
In order for this object to be achieved, the invention proposes a molding tool for producing hot-formed components, in particular formed sheet metal parts, having a tool lower part and a tool upper part which are movable in relation to one another, and which are configured with communicating operative faces for shaping the component. At least the operative face of a tool part is at least in portions configured so as to be coolable and cooling lines are provided in the tool part. At least one clearance which in a closed state of the tool conjointly with the component forms an air chamber which functions as a thermal insulator between the component and the tool part may be provided in at least one of the two tool parts. The subject matter of the invention is based on the effect of a workpiece that has been heated in a furnace to a temperature above Ac3, is moved in between the tool parts and is hot-formed by the direct method, while the workpiece simultaneously is geometrically formed and thermally cooled. In that region of the workpiece to which the clearance is adjacent, a medium that is trapped in the chamber serves as an insulator such that on the workpiece in the region of the chamber is cooled more slowly than in the remaining regions of the component. For this reason, a microstructure having lower mechanical strength is established in this region of the workpiece than in the remaining component. Gases, for example inert gases or ambient air, are particularly suitable as media.
The clearance may be configured so as to be variable in volume. For this reason, the amount of air in the chamber may be increased or decreased as required, and thus the level of insulation in relation to the cooled workpiece may be varied. Accordingly, better insulation is achieved as the greater the amount of air is present in the chamber.
An insert that configures a tool-side rear wall of the air chamber may be disposed in the tool part in which the clearance is configured. In other words, the insert has a surface portion that faces the component and that forms part of the wall of the clearance.
Furthermore, the insert may is disposed in the tool part so as to be movable, in particular displaceable in a linear manner. For this reason, depending on the desired degree of insulation, the insert may be transferred to a corresponding position, and the volume of the air chamber may be defined accordingly.
Moreover, ducts may be provided in the insert for controlling the temperature of the insert. The insert is preferably set to a predetermined temperature by way of a temperature-controlled medium, in particular by way of a gas or a liquid, for example oil or water. The medium herein is temperature-controlled in a heating and/or cooling installation and is directed through the ducts of the insert. This offers the advantage that the insert, as required, may be heated or cooled. For this reason, the volume in the chamber may likewise be temperature-controlled, thereby desired mechanical properties may be set in specific regions of the sheet metal part.
The insert in a closed state of the tool may be in a retracted position in order for the clearance to be generated. In this position, a chamber that is filled with a medium/air is then configured. The insert in an opened state of the tool may be in an advanced position in which the volume of the clearance is at least reduced in relation to the volume of the clearance in the retracted position of the insert. For this reason, the air is removed from the chamber, selectively after each forming procedure or after a predefined number of forming procedures, thereby the air in the chamber is prevented from being excessively heated. The clearance, or the chamber, respectively, is consequently regularly purged with the medium/air. The hot-forming procedure, and not least the microstructure in this region of the workpiece, is thus not compromised.
In an alternative embodiment, the clearance may be actively purged by way of a control unit or a regulating unit.
The formed sheet metal parts may be configured from a ferrous metal, in particular from steel, and/or from a non-ferrous metal, in particular from aluminum and/or magnesium.
The cooling lines may carry a coolant in a manner that is substantially parallel with a surface of the component. The coolant, which may be gaseous or liquid, is conducted in the cooling lines and does not come into direct contact with the component. The thermal energy of the coolant in the cooling lines acts through the material of the tool part on the component or the workpiece, and physically contacts the tool part during forming. Thus, individual regions of the component or of the workpiece may be cooled at variable speed or at variable intensity, and various mechanical properties may be implemented in the component for this reason.
Moreover, the cooling lines may be supplied with coolant by way of a common infeed system. This offers the advantage that a uniform temperature control or a supply of coolant of identical temperature is ensured in all cooling lines.
In a further alternative embodiment of the molding tool, the operative face of the tool upper part, and/or the operative face of the tool lower part may at least in portions be configured so as to be coolable. By virtue of the minor thickness of the sheet metal component that is in the range of 0.5 to 5 mm, unilateral cooling is sufficient. However, in order for particularly rapid and intense cooling to be implemented, it may be advantageous for bilateral cooling to be implemented.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.